This invention concerns new uniform polymeric filaments prepared by an improved process of melt-spinning at controlled high withdrawal speeds.
It has long been known that polymeric filaments, such as polyesters, can be prepared directly, i.e., in the as-spun condition, without any need for drawing, by spinning at high speeds of the order of 5 km/min or more. This was first disclosed by Hebeler in U.S. Pat. No. 2,604,667 for polyesters. There has been increased interest in the last 10 years, as shown by the number of patent specifications disclosing methods of melt-spinning at these high spinning speeds.
Frankfort et al. in U.S. Pat. Nos. 4,134,882 and 4,195,051 disclose new uniform polyester filaments and continuous filament yarns of enhanced dyeability, low boil-off shrinkage and good thermal stability, prepared by spinning and winding directly at withdrawal speeds of 5 km/min or more. The highest speed exemplified is 8000 ypm. The withdrawal speed is the speed of the first driven roll wrapped (at least partially) by the filaments, i.e., the feed roll. When uniform polymeric filaments are desired, such as are suitable for continuous filament yarns, for example, it is essential to use a roll or equivalent positive means, driven at a constant controlled speed to withdraw the filaments, as opposed to an air jet ejector. The latter is satisfactory for some uses, such as non-woven products, but does not produce filaments that are sufficiently uniform for use as continuous filament yarns for most purposes.
Tanji et al. U.S. Pat. No. 4,415,726 reviews several earlier references and discloses polyester filaments and yarns capable of being dyed under normal pressure, and a process for producing such polyester yarns with improved spinning stability at controlled high spinning (i.e., winding) speeds of at least 5 km/min. Sudden quenching and cross-flow quenching are avoided. The extruded filaments preferably pass through a heating zone of at least 150.degree. C. An important element is the subjection of the filaments to a vacuum or suction by an aspirator. This preferably gives the filaments a velocity of more than one tenth of the spinning speed. The heating zone and the aspirator are separated by a distance sufficient to avoid the filaments sticking together at the aspirator. The heating zone and the aspirator achieve high spinning efficiency and stability at high speed spinning. Tanji's examples 9-14 show the use of both heating zone and aspirator, while examples 1-7 show radial quench without any heating zone or aspirator. These examples produce polyester yarn having properties seemingly comparable to each other at respective speeds of 7, 8 and 9 km/min which latter is the highest winding speed used in the examples. Tanji do discuss the possibility of use of speeds up to 12 km/min.
Tanji do not explain why their polyester fibers have improved dyeability, but Shimizu et al. in a paper entitled "High Speed Spinning of Poly(ethylene terephthalate) Structure Development and Its Mechanism," given at the 22nd International Synthetic Fiber Symposium at Dornbirn in June, 1983, analogize an increase in dyeability with voids in the surface (sheath), which is consistent with a reduction in birefringence and mechanical properties. Shimizu et al. are among other experts who have noted that necking (neck-like deformation) take place when polyester filaments are spun at high speeds of the order of 5 km/min.
It would be very desirable from an economic viewpoint to melt-spin filaments and yarns having similar or better mechanical properties at even higher speeds, even if this would mean that the polyester products, for example, would have only the normal dyeability associated with conventional polyester filaments instead of any improved dyeability associated with the voids created by spinning as disclosed by Tanji et al. However, an article by Professor A. Ziabicki in Fiber World, September, 1984, pages 8-12, entitled "Physical Limits of Spinning Speed" questions whether higher speeds can yield fibers with better mechanical properties, and whether there are any natural limits to spinning speed which cannot be overcome (concentrating on physical and material factors only, and excluding economical and technical aspects of the problem). Professor Ziabicki concludes that there exists such a speed, beyond which no further improvement of structure and fiber properties is to be expected. In the case of polyester filaments studied in two references, referred to, the maxima appear to Professor Ziabicki to be around 5-7 km/min. This is consistent with the results shown by Tanji at speeds up to 9 km/min and by Shimizu.
Accordingly, it was very surprising to provide an improved process for obtaining polymeric filaments and yarns by melt-spinning at even higher speeds, without the accompanying deterioration in mechanical properties that has been shown and predicted in the prior art.
In contrast to Tanji's disclosure of preparing polymeric filaments by winding at high withdrawal speeds, with an aspirator to assist the withdrawal of the filaments from the spinneret, there have been several disclosures of preparing polymeric filaments by extruding into a pressurized chamber and using air pressure, e.g., an air nozzle or an aspirator to withdraw the filaments from the pressurized chamber without use of any winder or other positively-driving roll to advance the filaments at a controlled speed. The resulting filaments have many uses, especially in non-woven fabrics, but do not have the uniformity required for most purposes as continuous filament yarns, because of the inherent variability (along the same filament and between different filaments) that results from use of only an air jet to advance the yarns, i.e., without a winder or other controlled positive-driving mechanism. Indeed, the resulting filaments are often so non-uniform as to be spontaneously crimpable, which can be of advantage, e.g., for use in non-wovens, but is undesirable for other uses.